Abstract: A chemical metering and control system is provided which maintains a desired level of a pesticide (or other chemical) in any open or closed channel flow stream with fluctuating flow rates. The system dispenses an amount of the pesticide at an injection point of the flow stream along with a fluorescent indicating dye, and then takes a sample downstream from the injection point to determine how much of the indicating dye is present in the flow stream, and therefore how much pesticide is present. At the sampling point a pump extracts a flow stream sample and passes the sample to a fluorometer which measures the amount of fluorescent indicating dye injected upstream. A proportional integral derivative (PID) controller then adjusts the level of the pesticide/dye mixture in order to achieve the appropriate concentration of pesticide in the flow stream.